Perhaps the most important fact about polarization in Congress during the Obama years has been that Republican Members of Congress are frightened of primaries from their right while Democrats are frightened of being tarred as liberals in general elections. The result: Republicans have moved much farther away from the center than Democrats have — leading Dems to be far more willing to compromise on their priorities.

Could that change? Yesterday, two Democratic Members of the House were defeated in primaries in Pennsylvania by more liberal candidates. In the 12th District, where two Members were thrown together by redistricting, moderate Jason Altmire was defeated by labor favorite Mark Critz. And in the 17th, five-term Blue Dog Tim Holden was defeated by newcomer Matt Cartwright.

The question here is how these results, and any other moderate setbacks in other primaries this year, will be interpreted by Democratic politicians. Will they see it as just a couple of redistricting-inspired flukes? Or as a warning shot to Democratic elected officials who care more about avoiding the “liberal” label than they do about supporting policies that primary voters prefer?

Democratic politicians convinced of the latter interpretation will be far more likely to support the agenda urged on them by liberal activists.

Objectively, one can certainly argue that these were accidents of redistricting. Holton was clobbered by Republican-drawn lines, with only a small overlap between his old district and the new 17th. And while more of the 12th had been Altmire’s, he was still running against another Member, and losing a Member-Member race isn’t really anything like losing to a regular primary challenger.

On the other hand, in both battles liberals signed on with the winner, mainly to punish the losers for their Blue Dog moderation.

The question, then, is who will win the spin war over these nomination contests. When Republican voters ousted Bob Bennett in Utah in 2010, conservative party actors convinced every Republican politician that it could happen to them if they were labeled Republicans In Name Only, even if they were loyal to the party line on most votes. In part, this was the culmination of a multi-cycle campaign by the Club for Growth and other organized conservatives to target moderate Republicans with tough primary challengers. After the campaign in Utah and in other states with contested primaries won by right wing challengers, conservatives have always insisted that the winner won because of national conservative issues rather than any local factors or a general throw-the-bums-out feeling during a recession. The result? No Republican Member of Congress believes he or she is safe from primaries, and therefore does everything possible to avoid the RINO accusation.

If liberals want to duplicate this from the left, and make these two wins matter beyond the district lines, their work is cut out for them. They need to win the spin, and they need to keep it going in other districts this year and in future election cycles. The stakes are certainly high enough.

Imagine a scenario in which Democrats again win unified control of government — and instead of having to deal with dozens of Members who are terrified of voting for the mainstream liberal agenda, there are dozens of Members who are terrified of opposing it. Compared with 1993 or even 2009, that would be whole ‘nother ballgame.